[PATCH v2011.2] fs: symlink restrictions on sticky directories

From: Kees Cook
Date: Wed Dec 14 2011 - 19:47:28 EST

A long-standing class of security issues is the symlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like /tmp. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given symlink (i.e. a
root process follows a symlink belonging to another user). For a likely
incomplete list of hundreds of examples across the years, please see:
http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp

The solution is to permit symlinks to only be followed when outside
a sticky world-writable directory, or when the uid of the symlink and
follower match, or when the directory owner matches the symlink's owner.

Some pointers to the history of earlier discussion that I could find:

1996 Aug, Zygo Blaxell
http://marc.info/?l=bugtraq&m=87602167419830&w=2
1996 Oct, Andrew Tridgell
http://lkml.indiana.edu/hypermail/linux/kernel/9610.2/0086.html
1997 Dec, Albert D Cahalan
http://lkml.org/lkml/1997/12/16/4
2005 Feb, Lorenzo Hernández García-Hierro
http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html
2010 May, Kees Cook
https://lkml.org/lkml/2010/5/30/144

Past objections and rebuttals could be summarized as:

- Violates POSIX.
- POSIX didn't consider this situation and it's not useful to follow
a broken specification at the cost of security.
- Might break unknown applications that use this feature.
- Applications that break because of the change are easy to spot and
fix. Applications that are vulnerable to symlink ToCToU by not having
the change aren't. Additionally, no applications have yet been found
that rely on this behavior.
- Applications should just use mkstemp() or O_CREATE|O_EXCL.
- True, but applications are not perfect, and new software is written
all the time that makes these mistakes; blocking this flaw at the
kernel is a single solution to the entire class of vulnerability.
- This should live in the core VFS.
- This should live in an LSM. (https://lkml.org/lkml/2010/5/31/135)
- This should live in an LSM.
- This should live in the core VFS. (https://lkml.org/lkml/2010/8/2/188)

This patch is based on the patch in Openwall and grsecurity, along with
suggestions from Al Viro. I have added a sysctl to enable the protected
behavior, documentation, and a ratelimited warning.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Ingo Molnar <mingo@xxxxxxx>
---
v2011.2:
- Updated documentation, thanks to Randy Dunlap.
- Switched Kconfig default to "y", added __read_mostly to sysctl,
thanks to Ingo Molnar.
- Switched to audit logging to gain safe path and name reporting when
hitting the restriction.
v2011.1:
- back from hiatus
---
Documentation/sysctl/fs.txt | 21 +++++++++++
fs/Kconfig | 16 ++++++++
fs/namei.c | 85 ++++++++++++++++++++++++++++++++++++++++---
kernel/sysctl.c | 10 +++++
4 files changed, 126 insertions(+), 6 deletions(-)

diff --git a/Documentation/sysctl/fs.txt b/Documentation/sysctl/fs.txt
index 88fd7f5..4b47cd5 100644
--- a/Documentation/sysctl/fs.txt
+++ b/Documentation/sysctl/fs.txt
@@ -32,6 +32,7 @@ Currently, these files are in /proc/sys/fs:
- nr_open
- overflowuid
- overflowgid
+- protected_sticky_symlinks
- suid_dumpable
- super-max
- super-nr
@@ -157,6 +158,26 @@ The default is 65534.

==============================================================

+protected_sticky_symlinks:
+
+A long-standing class of security issues is the symlink-based
+time-of-check-time-of-use race, most commonly seen in world-writable
+directories like /tmp. The common method of exploitation of this flaw
+is to cross privilege boundaries when following a given symlink (i.e. a
+root process follows a symlink belonging to another user). For a likely
+incomplete list of hundreds of examples across the years, please see:
+http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
+
+When set to "0", symlink following behavior is unrestricted.
+
+When set to "1" symlinks are permitted to be followed only when outside
+a sticky world-writable directory, or when the uid of the symlink and
+follower match, or when the directory owner matches the symlink's owner.
+
+This protection is based on the restrictions in Openwall and grsecurity.
+
+==============================================================
+
suid_dumpable:

This value can be used to query and set the core dump mode for setuid
diff --git a/fs/Kconfig b/fs/Kconfig
index 5f4c45d..26ede24 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -278,3 +278,19 @@ source "fs/nls/Kconfig"
source "fs/dlm/Kconfig"

endmenu
+
+config PROTECTED_STICKY_SYMLINKS
+ bool "Protect symlink following in sticky world-writable directories"
+ default y
+ help
+ A long-standing class of security issues is the symlink-based
+ time-of-check-time-of-use race, most commonly seen in
+ world-writable directories like /tmp. The common method of
+ exploitation of this flaw is to cross privilege boundaries
+ when following a given symlink (i.e. a root process follows
+ a malicious symlink belonging to another user).
+
+ Enabling this solves the problem by permitting symlinks to be
+ followed only when outside a sticky world-writable directory,
+ or when the uid of the symlink and follower match, or when
+ the directory and symlink owners match.
diff --git a/fs/namei.c b/fs/namei.c
index 5008f01..ed2747a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -624,10 +624,78 @@ static inline void put_link(struct nameidata *nd, struct path *link, void *cooki
path_put(link);
}

+int protected_sticky_symlinks __read_mostly =
+#ifdef CONFIG_PROTECTED_STICKY_SYMLINKS
+ 1;
+#else
+ 0;
+#endif
+
+/**
+ * may_follow_link - Check symlink following for unsafe situations
+ * @dentry: The inode/dentry of the symlink
+ * @nameidata: The path data of the symlink
+ *
+ * In the case of the protected_sticky_symlinks sysctl being enabled,
+ * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is
+ * in a sticky world-writable directory. This is to protect privileged
+ * processes from failing races against path names that may change out
+ * from under them by way of other users creating malicious symlinks.
+ * It will permit symlinks to be followed only when outside a sticky
+ * world-writable directory, or when the uid of the symlink and follower
+ * match, or when the directory owner matches the symlink's owner.
+ *
+ * Returns 0 if following the symlink is allowed, -ve on error.
+ */
+static inline int
+may_follow_link(struct dentry *dentry, struct nameidata *nameidata)
+{
+ int error = 0;
+ const struct inode *parent;
+ const struct inode *inode;
+ const struct cred *cred;
+
+ if (!protected_sticky_symlinks)
+ return 0;
+
+ /* Allowed if owner and follower match. */
+ cred = current_cred();
+ inode = dentry->d_inode;
+ if (cred->fsuid == inode->i_uid)
+ return 0;
+
+ /* Check parent directory mode and owner. */
+ spin_lock(&dentry->d_lock);
+ parent = dentry->d_parent->d_inode;
+ if ((parent->i_mode & (S_ISVTX|S_IWOTH)) == (S_ISVTX|S_IWOTH) &&
+ parent->i_uid != inode->i_uid) {
+ error = -EACCES;
+ }
+ spin_unlock(&dentry->d_lock);
+
+#ifdef CONFIG_AUDIT
+ if (error) {
+ struct audit_buffer *ab;
+
+ ab = audit_log_start(current->audit_context,
+ GFP_ATOMIC, AUDIT_AVC);
+ audit_log_format(ab, "op=follow_link action=denied");
+ audit_log_d_path(ab, "path=", &nameidata->path);
+ audit_log_format(ab, " name=");
+ audit_log_untrustedstring(ab, dentry->d_name.name);
+ audit_log_format(ab, " dev=%s ino=%lu",
+ inode->i_sb->s_id,
+ inode->i_ino);
+ audit_log_end(ab);
+ }
+#endif
+ return error;
+}
+
static __always_inline int
-follow_link(struct path *link, struct nameidata *nd, void **p)
+follow_link(struct path *link, struct nameidata *nd, void **p, bool sensitive)
{
- int error;
+ int error = 0;
struct dentry *dentry = link->dentry;

BUG_ON(nd->flags & LOOKUP_RCU);
@@ -646,7 +714,10 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
touch_atime(link->mnt, dentry);
nd_set_link(nd, NULL);

- error = security_inode_follow_link(link->dentry, nd);
+ if (sensitive)
+ error = may_follow_link(link->dentry, nd);
+ if (!error)
+ error = security_inode_follow_link(link->dentry, nd);
if (error) {
*p = ERR_PTR(error); /* no ->put_link(), please */
path_put(&nd->path);
@@ -1339,7 +1410,7 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
struct path link = *path;
void *cookie;

- res = follow_link(&link, nd, &cookie);
+ res = follow_link(&link, nd, &cookie, 0);
if (!res)
res = walk_component(nd, path, &nd->last,
nd->last_type, LOOKUP_FOLLOW);
@@ -1612,7 +1683,8 @@ static int path_lookupat(int dfd, const char *name,
void *cookie;
struct path link = path;
nd->flags |= LOOKUP_PARENT;
- err = follow_link(&link, nd, &cookie);
+
+ err = follow_link(&link, nd, &cookie, 1);
if (!err)
err = lookup_last(nd, &path);
put_link(nd, &link, cookie);
@@ -2324,7 +2396,8 @@ static struct file *path_openat(int dfd, const char *pathname,
}
nd->flags |= LOOKUP_PARENT;
nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
- error = follow_link(&link, nd, &cookie);
+
+ error = follow_link(&link, nd, &cookie, 1);
if (unlikely(error))
filp = ERR_PTR(error);
else
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index ae27196..cc2c5f9 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -92,6 +92,7 @@ extern int sysctl_overcommit_memory;
extern int sysctl_overcommit_ratio;
extern int max_threads;
extern int core_uses_pid;
+extern int protected_sticky_symlinks;
extern int suid_dumpable;
extern char core_pattern[];
extern unsigned int core_pipe_limit;
@@ -1495,6 +1496,15 @@ static struct ctl_table fs_table[] = {
#endif
#endif
{
+ .procname = "protected_sticky_symlinks",
+ .data = &protected_sticky_symlinks,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
+ {
.procname = "suid_dumpable",
.data = &suid_dumpable,
.maxlen = sizeof(int),
--
1.7.0.4

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